A pump operates by drawing fluid at a low pressure from a suction line into a pump inlet and propelling fluid out of a pump outlet at a higher pressure or velocity. For proper operation each pump requires a net positive suction head (NPSH-R) which is the equivalent total head of liquid at the pump centerline less the vapor pressure of the liquid at the pump centerline. The pump manufacturer establishes the NPSH-R required for each pump. In an installation, the available net positive suction head (NPSH-A) must be equal to or greater than the NPSH-R of the pump. If the NPSH-A is not adequate, the pump may cavitate. Cavitation at start up may prevent the pump from pumping and may cause damage to the pump parts. Fluids near their boiling point have lower NSPH-A, making them difficult to pump. When the fluid in the suction line is colder than the ambient temperature, the heat leak into the suction line will warm the fluid and further reduce the NPSH.
Various methods are used to increase the NPSH-A in a pumping system where the fluid is at or near its boiling point. Insulating the suction line decreases the rate of heat leak to the fluid, but if the pump is not operating, the heat leak will eventually cause the fluid to boil and the NPSH available to the pump at start up will be near zero. Unless the pump NPSH-R requirement is zero, the pump at start up will cavitate and cause a pumping failure, regardless of the head in a reservoir from which the fluid is pumped. Thus, after inactive periods, the initial pump start-up may be difficult or impossible.